1. Field of the Invention
The present invention relates to an electrode which emits electrons when a high electric field is applied to it, a method of manufacturing the electrode, and an electron-emitting device incorporating the electrode.
2. Description of the Related Art
Electrodes are known which emit electrons when a high electric filed is applied to them. These electrodes, generally known as "electron-emitting electrodes," are shaped like a needle or a plate, unlike hot cathodes which are coiled filaments and which emit electrons when a current flows through them to heat them. An electron-emitting electrode emits electrons from its surface by virtue of tunnel effect when a high voltage of about 10.sup.7 V/cm or more is applied to it. They are used as cathodes in copying apparatuses, in cold-cathode fluorescent lamps employed as the back lights of non-self-emission displays (e.g., liquid crystal displays), in monochromatic or color display displays, in a plasma displays, in VFDs (Vacuum Fluorescent Displays), and in similar apparatuses.
A cold-cathode fluorescent lamp equipped with an electron-emitting electrode comprises a tube which has a fluorescent layer coated on its inner surface and which is filled with a mixture rare gas and mercury vapor. The electrons emitted from the electrodes impinge upon the mercury atoms in the tube so that, ultraviolet rays are generated. The ultraviolet rays excite the fluorescent layer. Thus excited, the fluorescent layer emits visible light.
Electron-emitting electrodes are made of metal having a small work function, such as nickel (Ni), molybdenum (Mo) or the like. Generally, the smaller the diameter of a cold-cathode fluorescent lamp having an electron-emitting electrode made of such metal, the higher the luminance (cd/m.sup.2) of the light the lamp generates. The cold-cathode fluorescent lamp can be thin and can emit intense light. It is therefore suitable for use as a back light in liquid crystal displays.
However, the voltage applied to the cold-cathode fluorescent lamp must be increased to enable the lamp to emit light of higher luminance. Consequently, the lamp consumes much power. If the lamp is used in a battery-driven portable display, the display cannot operate for a long time. The cold-cathode fluorescent lamp with electron-emitting electrodes is disadvantageous in another respect. As discharge proceeds, the material of electrode is gradually sputtered, inevitably contaminating the inner surface of the tube and shortening the lifetime of the electron-emitting electrode.
Various materials of electron-emitting electrodes have been proposed. Various limitations are imposed on the material for an electron-emitting electrode, however. For example, insulating materials that greatly prevent tunneling cannot be used in a cold cathode wherein tunnel effect is utilized.